1. Field of the Invention
The present invention relates to a system for reproducing information which is recorded on a recording medium.
2. Description of the Prior Art
A system is well known in which information is recorded on and read out from a recording medium by utilizing a beam of light. Various types of recording mediums such as discs, cards, tapes and the like may be employed in this system. In particular, a card-shaped optical recording medium (hereinafter referred to as an "optical card") can be formed in a small-size, lightweight and portable medium having a large storage capacity. Therefore, the demand for this type of medium has increased.
The recording of information on the optical card is practiced in the following manner. A light beam is modulated in accordance with information to be recorded and the modulated beam is converged in the form of a minute beam spot. The thus-converged beam is caused to scan the surface of the card, whereby the information is recorded on the card in the form of a row of recording pits (i.e., an information track) which is optically detectable.
In the case of the above-mentioned procedure for information recording, it is necessary to perform accurate recording and reproduction of information without involving any trouble such as a light beam crossing the information track. To this end, in general, a track guide for facilitating automatic tracking, clock signals used for correcting variations in velocity during a recording operation, and a track number for seeking a desired track are disposed in advance adjacent to each of the information tracks as a guide means. According to this arrangement, at the time of reproduction the clock signals and the track number are reproduced, and the signals thus obtained are binary-coded into digital signals. On the basis of the digital signals, desired data is reproduced from a predetermined information track and the reproduced data is extracted in the form of binary-coded information.
As a typical conventional case, the predetermined arrangement of the track guides, the clock signals and the track numbers on a medium (i.e., preformat) has been effected at an intermediate point in the course of the manufacture of the recording medium. However, occasionally the reproduced characteristics of the preformatted information are different from the characteristics obtained from the reproduction of the information recorded by another recording/reproducing system, so that if the clock signals, the track numbers and recorded data are to be read out by an identical means used in common, it is impossible accurately to reproduce information.
In case of the optical card, preformatting is practiced by photo-lithography, and when information is to be recorded by a recording system, a laser beam is irradiated on a medium and record pits are formed thereon by heat. In terms of optical properties, the former is of a phase type in which variations are detected in the quantity of light caused by scattered reflection, and the latter is of a reflection type in which variations are detected in the quantity of light caused by the difference in reflectivity. Therefore, as shown in FIGS. 5(a.sub.1) and 6(d.sub.1), each type exhibits a different reproduced waveform. FIG. 5(a.sub.1) shows an example of the waveform obtained by the reproduction of the preformatted portion, and FIG. 6(d.sub.1) shows that obtained by the reproduction of one of the record pits.
Even in the case of a waveform obtained by the reproduction of preformat, as shown in FIG. 5(a.sub.2), a level difference occurs due to the individual differences of each medium. Similarly, the reproduced waveform of each record pit occasionally undergoes distortion since a non-uniform portion is formed in the vicinity of the boundary portion defining the record pit or at the bottom of any of the pits owing to the individual differences of each medium or to variations in the focused state of a laser beam during a recording operation.
For this reason, if binary-coding is to be carried out by an identical means, a readout margin may be damaged, leading to difficulties such as an inferior S/N ratio and erroneous reproduction.